In the aerospace industry, engine components, particularly engine casings, are conventionally formed by ring-roll forging. For example compressor casings are formed from titanium alloys, such as Ti 6Al4V (6-4), however other materials such as steel and nickel may be used in other parts of the engine.
Conventional ring-roll forged components are first forged to approximately the desired shape but must be heavily machined in order to achieve the final component. This final machining operation dramatically increases the cost per component due to the increased labour and reduced material utilisation.
It is desirable to form components that are net-shape or near net-shape in order to remove or reduce the final machining process and thus reduce the cost per component.
One method of producing net shape or near net-shape components is by using powder metallurgy. Powder metallurgy is where a powdered metal is consolidated by applying pressure and/or high temperatures over time to form a component.
There are various techniques known in the art for consolidating a powder, an example of which is isostatic pressing. Isostatic pressing differs from other powder metallurgy techniques in that the powder is compacted and consolidated without direct contact with the pressuring medium. This is achieved by placing the powder within a container, conventionally known as a canister, which acts as a barrier between the pressuring medium and the powder. The canister is typically evacuated and sealed and the powder out-gassed to avoid any contamination of the powder. The canister is deformed by the pressurising medium and thus transfers the pressure to the powder without direct contact. As a result of this configuration, isostatic pressing creates a uniform pressure on the powder which creates a consolidated component with a homogenous density.
Isostatic pressing can be categorised into two separate types: cold isostatic pressing (CIP) and hot isostatic pressing (HIP).
In CIP, the canister is typically made from a flexible material such as a rubber. The pressurising medium, usually a liquid, is applied at room temperature to compact the powder. The compacted powder must be subsequently sintered to bond the particles to one another.
HIP differs from CIP in that the powder is effectively compacted and sintered simultaneously by pressurising and also heating the powder. The canister is usually a metallic container although glass may be used, and the pressurising medium is a gas, typically an inert gas such as argon or helium. Due to the high temperatures, the canister plastically deforms under the applied pressure which is transferred to the powder thus consolidating the powder to form the component.
Following consolidation of the powder it is necessary to remove the canister from the component. This may be achieved by machining the canister from the component or by applying an acid solution to the canister. This additional processing increases the cost of the component and the waste acid solution is undesirable from an environmental point of view.
Furthermore, isostatic pressing techniques require complex computer modelling to determine the correct geometry for the canister and pre-isostatic pressing powder profile. This again increases the cost per component.
The present invention provides a method and assembly for isostatic pressing a net-shape or near net-shape component which addresses the above problems associated with conventional techniques.